This invention relates to a device for testing circuits and circuit breakers.
The device disclosed in U.S. Pat. No. 5,736,861 is useful. However, several additional features would help with the functionality of the device.
Accordingly, this invention features several improvements to the circuit tester disclosed in U.S. Pat. No. 5,736,861, the disclosure of which is incorporated herein by reference.
One improvement is to control the branch circuit shorting time based on the branch impedance. The impedance measurement is made as described in the patent, column 3, lines 25-67. The shorting time referenced in the ""861 patent in column 2, lines 63-67, is 0.1 to 10 seconds. This is a predetermined, fixed time, established by the RC circuit values. The improvement of this aspect of the invention is to make the shorting test time proportional to the measured branch impedance. The proportionality can be linear, or established though some other arithmetic algorithm. If the impedance is low, then the test time is short. If the impedance is higher, then the test time is longer. The variable shorting test time is under control of microprocessor 66.
The variable test time would preferably be implemented in the firmware contained in memory serving the microprocessor. The firmware would take the branch impedance measured in the pre qualification test (""861 patent, column 3 lines 25-32). If the branch impedance was within certain upper and lower limits, a shorting time would be calculated from either an algorithm like a simple, linear or exponential proportionality, or a look up table contained in the firmware. The hardware remains the same as defined in the ""861 patent.
The second improvement herein contemplates the measurement of any change in current flow during the shorting test. Since a change in current flow may be caused by a fault in the branch circuit, if the current changes too rapidly, the breaker test may be aborted. An acceptable change in current (thus energy dissipated, which causes a temperature change in the circuit) can be predicted from the circuit configuration (e.g., measured branch impedance). Measurements outside of an acceptable current flow window may indicate a defective circuit.
In order to implement this aspect of the invention, a current sensing transformer 74 is added to the test device disclosed in the ""861 patent. An analog to digital converter 76 would also be added to convert the measured analog current output from the sensor to a digital data stream for the microprocessor. The current sensing transformer can be accomplished by using the existing wire going to the fuse in the ""861 patent as the primary wire, with the transformer secondary accomplished with one or more turns, terminated by the A/D converter. The digital signal from the A/D converter would be examined by the firmware in the microprocessor many times during the shorting period. If there was a drop in sampled current in excess of a predetermined amount, then the test would be aborted, for example by the microprocessor causing the relay to de-energize, and an error signal would be sent to the user interface.
The third improvement herein contemplates the use of arc detection added to the breaker tester. The arc detection can be used to qualify a circuit prior to the shorting test. Also, once the circuit is qualified, arc detection can be used to monitor the circuit for any arcing during the shorting test. In the ""861 patent, there is already a voltage sensor 50. With the current flow concept described above, a current sensor 74 is added. These two items comprise the additional hardware needed to accomplish arc detection. For arc detection, the current sensor and the voltage sensor both need to be xe2x80x9chigh speedxe2x80x9d, in that they must be able to detect voltage and current variations that happen quickly. As an example, the detection should take place at a bandwidth of least 500 KHz in order to capture the frequencies associated with arc signatures. Other than the addition of xe2x80x9chigh speedxe2x80x9d current and voltage sensing as described, the hardware is unchanged and the arc detection is accomplished in the firmware.
A simple method to accomplish arc detection would be to sample both the voltage and current sensors, and if significant energy above the fundamental power line frequency (60 Hz in the US) was detected, then the test would either not be performed (if in the qualification phase), or, if in the test phase, the test would be aborted. The firmware would determine the ratio of fundamental power line energy to high frequency energy, which is indicative of arcing. The ratio would be determined preferably using preset limits stored in the processor memory. Alternatively, arc detection can be accomplished by comparing the waveshape, or measurements of the waveshape, to a stored library of known arc signatures. This would also be done in the firmware of the microprocessor.
This invention features in one embodiment an improved circuit breaker test device for testing a circuit breaker and the branch circuit of which the circuit breaker is a part, the circuit breaker test device comprising first switch means for controllably short circuiting the branch circuit; control means for causing the first switch means to short circuit the branch circuit; timer means, responsive to operation of the control means, for causing the first switch means to cease short circuiting the branch circuit after a test time; and test means for determining an impedance value of the branch circuit, and enabling operation of the control means only if the impedance value is within predetermined limits, the improvement comprising the timer means capable of establishing a variable test time. The test time may be established to be proportional to the determined impedance value of the branch circuit. The test time may be established in order to maintain a constant energy carried by the branch circuit during the short circuit test.
Also featured is an improved circuit breaker test device for testing a circuit breaker and the branch circuit of which the circuit breaker is a part, the circuit breaker test device comprising first switch means for controllably short circuiting the branch circuit; control means for causing the first switch means to short circuit the branch circuit; timer means, responsive to operation of the control means, for causing the first switch means to cease short circuiting the branch circuit after a test time; and test means for determining an impedance value of the branch circuit, and enabling operation of the control means only if the impedance value is within predetermined limits, the improvement comprising means for measuring the current flowing in the branch circuit during the short circuit test.
The improved circuit breaker test device of this same embodiment may further comprise means, responsive to the means for measuring current, for disabling operation of the control means upon measurement of a predetermined change in the measured current. The predetermined change may be a drop in the current in excess of a predetermined amount.
The improved circuit breaker test device may in this same embodiment still further comprise means for measuring the voltage in the branch circuit. The improved circuit breaker test device of this same embodiment may further comprise means for detecting arcing in the branch circuit. The means for detecting arcing may then comprise means, responsive to the means for measuring current and the means for measuring voltage, for detecting energy above the fundamental frequency. The means for detecting arcing may further comprise means for determining the ratio of energy at the fundamental frequency to energy above the fundamental frequency.